1. Field of the Invention
This invention concerns a high pole count motor having an integrated absolute magnetic position feedback, which, with related electronics, is capable of providing absolute position sensing over an arbitrary number of rotations of the motor. The multi-revolution absolute position feedback may be combined with a high resolution partial revolution feedback to provide high resolution feedback over an arbitrary number of rotations of the motor. The magnetic circuits of both the multiple revolution capable feedback and the high resolution partial revolution feedback are based on the existing motor magnetic circuits.
2. Description of the Related Art
Motors are used in a variety of applications for effecting the controlled motion of objects. For example, motors are used in a variety of industrial automation and other automation applications. In many applications, it is useful to provide the motor or the motor's controller with an accurate measure of the motor position to allow for greater precision in positioning objects with the motor. In many applications it is advantageous to know the absolute position of the motor upon application of power, even if the motor has been moved while system power was turned off. In closed loop systems, one or more sensors collect position, velocity or acceleration information about a motor and provide that information to the motor controller. A closed-loop control system within the motor controller receives the motor position or other information as feedback and improves the accuracy of the positioning or movement characteristics of the motor. Motors whose operation is affected by feedback and under closed loop control are often referred to as servomotors.
Absolute multi-turn position feedback can be obtained by a number of methods, including the use of multiple encoder sections or resolver sections with mechanical gearing between them, the use of sensors with battery backup and electronic counters, and the use of magnetic pulse generators to sense position movement and, optionally, to provide the energy needed to count the number and sequence of pulses so as to track shaft position.
The resolution of the absolute position sensors is often improved by combining the value of a coarse absolute position sensor with those of a fine position sensing method that provides an absolute position within a limited period or portion of a revolution. Provided that the coarse position sensing method is able to determine the position to within a fraction of a cycle of the fine position sensing method, the data from these two sensors may be combined by electronic means to provide high resolution position feedback over a wide absolute range of positions.
The combination of higher resolution cyclic absolute position sensors with coarse multi-turn absolute position sensors is well known in the art. Wong in U.S. Pat. No. 7,579,829 describes the combination of high resolution absolute-cyclic position sensing with lower resolution multi-turn absolute position sensing. The lower resolution absolute position sensing is obtained by using multiple resolvers coupled together by mechanical gearing. Shibata et al. in US Patent Application Pub. No. US 2011/0156699 A1 describes a similar multi-turn absolute position sensing method using multiple resolver sections mechanically coupled through gearing. There are multiple patents based on variations of the mechanical gearing method.
Jones in U.S. Pat. No. 5,057,727 describes a “Shaft Position Sensor Employing a Wiegand-effect Device.” In particular, U.S. Pat. No. 5,057,727 teaches the use of multiple Wiegand effect sensors to determine the coarse position of a toothed magnetically soft wheel, with the magnetic field needed for the Wiegand effect device provided by a non-wheel mounted magnetic source. The non-wheel mounted magnetic source may either be a permanent magnet or a solenoid. The phasing of the signals from the multiple Wiegand sensors provides incremental position information. The operation of the Wiegand wire, which is the basis of the Wiegand effect sensors, is described in U.S. Pat. No. 3,820,090 to Wiegand as well as in U.S. Pat. No. 4,247,601 to Wiegand. The Wiegand sensor is comprised of a Wiegand wire surrounded by a coiled sensing wire. The sensing wire produces a strong distinct pulse when the magnetized state of the inner core of the Wiegand wire is abruptly reversed upon the weakening or reversal of the applied magnetic field following the application of a sufficiently strong magnetic field to the sensor. Sensors based on this effect are well known in the art including U.S. Pat. No. 4,538,082 to Hinke et al.
Mehnert et al. in U.S. Patent Application Pub. No. US 2010/0213927 describe an absolute magnetic position encoder using a coarse magnetosensitive position sensor, such as a Hall effect sensor, sensing multiple permanent magnets of alternating polarity affixed to the body to be monitored, with the power to operate the control logic and non-volatile counters provided by a single Wiegand element. This coarse position sensor is combined with a magnetosensitive fine sensor, such as a Hall effect sensor, to provide a higher resolution portion. These are combined in electronic logic to provide a high resolution multi-turn encoder that does not require external power to maintain its absolute position count.
Menhert et al. in U.S. Pat. No. 8,111,065 describe a combination Wiegand effect sensor driven by a gear from the main shaft to produce multiple power pulses per revolution, with the pulses being counted to provide a coarse absolute position means. An absolute one turn encoder, resolver, or Hall effect position sensor means is used to provide a higher resolution means with the high resolution cyclic and the low resolution absolute count combined to provide an absolute position sensing means. The pulses from the Wiegand effect device are used to power the coarse absolute count electronics so that no external electrical power is required to track the coarse motion of the input shaft.
Mehnert in U.S. Patent Application Pub. No. US 2011/0006757 A1 describes the combination of a single Wiegand effect pulse generator contained within a ferro-magnetic ring with affixed magnets on the inner surface to generate alternating magnetic fields to the Wiegand effect device. An additional sensor is used to determine the direction of motion to generate the signals to the up/down counter for the coarse position, as well as functioning as the fine resolution portion of the position feedback.
Mehnert et al. in U.S. Patent Application Pub. No. US 2011/0184691 further teaches the electronics needed to process and combine the absolute position sensor described in U.S. Patent Application Pub. No. US 2011/0006757. For the coarse sensor, a single Wiegand effect sensor is combined with a Hall effect type device, with the Hall effect device powered from the Wiegand effect sensor. The hysteresis of the Wiegand device is used advantageously with the Hall effect device to determine the direction of rotation, as the hysteresis involved with the Wiegand device causes the Wiegand device signal to be out of phase with respect to the magnetic field as measured by the Hall effect device, according to the direction of rotation.
Zägelein et al. in U.S. Pat. No. 4,779,075 describes a device for absolute displacement determination using three Wiegand or large Barkhausen effect devices using non-volatile memory to determine actions based on prior pulses and a counter to store cycle counts, combined with an absolute-over-single-revolution position sensor attached to the shaft of the revolution counter. The center Wiegand device (S3) in U.S. Pat. No. 4,779,075 is used as a pre-trigger enable, and then the first pulse to either of the other Wiegand devices (S2 or S1 as identified in U.S. Pat. No. 4,779,075) is used to operate the counter up or down. In this manner, multiple pulses from the same Wiegand device are ignored if they occur without sufficient rotation to first engage S3.
Morita in U.S. Pat. No. 5,663,641, teaches a rotational speed detection unit with a Wiegand effect or amorphous magnetostriction wire sensor magnetically coupled between alternating poles of a tone wheel. The outer periphery of the tone wheel has alternative N and S poles, with a uniform pitch, such that the detection unit spans between a pair of magnets of opposing polarization. The polarities that are spanned alternate as the wheel is rotated, producing a series of pulses.